1. Technical Field
The present invention relates to an oscillating circuit which is driven by a constant current (constant current driven oscillating circuit), and in particular to a constant current driven oscillating circuit employed in a clock or the like of an LSI.
2. Related Art
Conventionally known is a constant current liquid crystal oscillating circuit that, in order to lower power consumption, employs an oscillation stop control signal, and stops oscillating operation of the oscillating circuit during an oscillation stop mode (see Japanese Patent Application Laid-Open (JP-A) No. 61-225904).
Furthermore, in order to lower power consumption, there is also a liquid crystal oscillating circuit that, by lowering a back gate electric potential of an N-channel field effect transistor (referred so as an N-MOSFET below) of a CMOS inverter when power is introduced using a switching circuit, thereby lowering the threshold voltage, has speedy start up of the liquid crystal oscillating circuit, from when power is introduced up to the point when a stable oscillating state is achieved, and then after stable oscillation has been achieved in the liquid crystal oscillating circuit, raises the threshold voltage by raising the back gate electric potential of the N-MOSFET, thereby reducing power consumption (JP-A No. 2001-298326).
Furthermore, there is an oscillating circuit that, in order to perform oscillating operation at a low power source voltage, lowers a gate voltage input to a gate terminal of a P-channel field effect transistor (referred to below as a P-MOSFET) of a CMOS inverter, using an voltage lowering circuit, with an accompanying increase in the drain current (JP-A No. 6-97732).
Furthermore, there is a constant current driven liquid crystal oscillating circuit that makes a gate signal for input to the gate terminal of a P-MOSFET of a CMOS inverter constant, and makes the drain current flowing of the P-MOSFET constant by the P-MOSFET constantly being in a conducting state, and suppresses the penetration current that occurs when the P-MOSFET and an N-MOSFET are both conducting at the same time (JP-A No. 2008-219387).
However, the constant current liquid crystal oscillating circuit described in JP-A No. 61-225904 is merely a technique to reduce power consumption by stopping the oscillating circuit when oscillating operation is not required, and cannot reduce power consumption during oscillating operation.
Moreover, the liquid crystal oscillating circuit described in JP-A No. 2001-298326 uses a switching circuit to raise the back gate electric potential of the N-MOSFET of a CMOS inverter and to raise the threshold voltage of the N-MOSFET, and therefore the circuit configuration is complicated.
Furthermore, the oscillating circuit described in JP-A No. 6-97732 merely performs oscillating operation at a low power source voltage, and when the N-MOSFET of the CMOS inverter is in the ON state, the drain current that flows to the N-MOSFET via the P-MOSFET is wasted.
Furthermore, the constant current driven oscillating circuit described in JP-A No. 2008-219387 merely makes the drain current flowing to the P-MOSFET of the CMOS inverter constant and suppresses the penetrating current, and similar to in JP-A No. 6-97732, when the N-MOSFET is in the ON state, the drain current flowing to the N-MOSFET via the P-MOSFET is wasted.